Glass laminate with buried stress spikes to arrest cracks and methods of making the same

ABSTRACT

A laminated glass article comprises a core layer comprising a core glass composition, and a cladding layer directly adjacent to the core layer and comprising a clad glass composition. A stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from a compressive stress to a tensile stress, transitions to a compressive stress as a step-change at an interface region between the core layer and the cladding layer, and increases with increasing distance from the interface region to a center of the core layer from the compressive stress to a tensile stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 17/632,086, filed on Feb. 1, 2022, which claims the benefit of priority under 35 U.S.C. § 371 of International Application No. PCT/US2020/044436, filed on Jul. 31, 2020, which claims the benefit of priority under 35 U.S.C § 119 of U.S. Provisional Application Ser. No. 62/883,401, filed on Aug. 6, 2019, the content of each of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND Field

The present specification generally relates to glass articles and, more particularly, to laminated glass articles including a plurality of glass layers and methods for forming the same.

Technical Background

Glass articles can be used in a wide variety of products, including, for example, auto-glazing, architectural panels, appliances, and cover glass (e.g., for touch-screen devices such as smartphones, tablets, laptop computers, and monitors). Relatively large flaws can be introduced into the surfaces of the glass articles during use. For example, when a smartphone is dropped on a rough surface, such as asphalt, the local indention caused by contact with sharp features of the rough surface can cause flaws as deep as about 300 μm in the surface of the cover glass.

Accordingly, a need exists for glass articles with improved resistance to breakage caused by deep flaws, improved mechanical reliability, and improved drop performance.

SUMMARY

According to a first aspect disclosed herein, a laminated glass article comprises a core layer comprising a core glass composition, and a cladding layer directly adjacent to the core layer and comprising a clad glass composition. A stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from a compressive stress to a tensile stress, transitions to a compressive stress as a step-change at an interface region between the core layer and the cladding layer, and increases with increasing distance from the interface region to a center of the core layer from the compressive stress to a tensile stress.

According to a second aspect, a laminated glass article includes the laminated glass article according to the first aspect, wherein the core glass composition has an average core coefficient of thermal expansion (CTE_(core)) and the cladding layer has an average cladding coefficient of thermal expansion (CTE_(clad)) that is greater than the CTE_(core).

According to a third aspect, a laminated glass article includes the laminated glass article according to the first or second aspects, wherein the laminated glass article is ion-exchange strengthened.

According to a fourth aspect, a laminated glass article includes the laminated glass article according to any preceding aspect, wherein the core glass composition and the clad glass composition comprise one or more alkali oxides.

According to a fifth aspect, a laminated glass article includes the laminated glass article according to any preceding aspect, wherein a maximum tensile stress in the cladding layer is greater than or equal to a maximum tensile stress in the core layer.

According to a sixth aspect, a laminated glass article includes the laminated glass article according to any preceding aspect, wherein a maximum compressive stress of the core layer is greater than a maximum compressive stress of the cladding layer.

According to a seventh aspect, a laminated glass article includes the laminated glass article according to any preceding aspect, wherein a magnitude of the stress at the outer surface is greater than or equal to about 300 MPa.

According to an eighth aspect, a laminated glass article includes the laminated glass article according to any preceding aspect, wherein a maximum magnitude of the stress of the laminated glass article is less than or equal to about 400 MPa.

According to a ninth aspect, a laminated glass article comprises a core layer comprising a core glass composition, and a cladding layer directly adjacent to the core layer and comprising a cladding glass composition. A concentration of Li ions increases with increasing distance from an outer surface of the cladding layer to a centerline of the core layer; a concentration of K ions decreases with increasing distance from the outer surface of the cladding layer to an interface between the cladding layer and the core layer; and a concentration of Na ions increases with increasing distance from the outer surface of the cladding layer to a maximum concentration of Na ions in the cladding layer or at the interface between the cladding layer and the core layer and transitions to a lower concentration of Na ions relative to the maximum concentration of Na ions via a step change at the interface between the cladding layer and the core layer.

According to a tenth aspect, a laminated glass article includes the laminated glass article according to the ninth aspect, wherein the concentration of Na ions decreases with increasing distance from the interface between the cladding layer and the core layer to the centerline of the core layer.

According to an eleventh aspect, a laminated glass article includes the laminated glass article according to the ninth aspect, wherein the concentration of Na ions increases and then decreases with increasing distance from the interface between the cladding layer and the core layer to the centerline of the core layer.

According to a twelfth aspect, a laminated glass article includes the laminated glass article according to any of the ninth through eleventh aspects, wherein the concentration of Li ions increases according to a non-linear relationship.

According to a thirteenth aspect, the laminated glass article includes the laminated glass article according to any of the ninth through twelfth aspects, wherein a concentration of K ions in the core layer is zero.

According to a fourteenth aspect, the laminated glass article includes the laminated glass article according to the ninth aspect, wherein the concentration of Na ions increases from the outer surface of the cladding layer to the maximum concentration of Na ions according to a linear relationship.

According to a fifteenth aspect, the laminated glass article includes the laminated glass article according to the ninth aspect, wherein the concentration of Na ions increases from the outer surface of the cladding layer to the maximum concentration of Na ions according to a non-linear relationship.

According to a sixteenth aspect, a method of preparing a laminated glass article comprises laminating at least one core layer and at least one cladding layer to form a laminated glass article, the at least one core layer comprising an ion-exchangeable core glass composition and the at least one cladding layer comprising an ion-exchangeable clad glass composition; and contacting the laminated glass article with an ion exchange bath comprising at least one ion source for a time such that a maximum compressive stress in the at least one core layer is greater than a maximum compressive stress in the at least one cladding layer.

According to a seventeenth aspect, a method comprises the method according to the sixteenth aspect, wherein the at least one core layer has an average core coefficient of thermal expansion (CTE_(core)) and the at least one cladding layer has an average cladding coefficient of thermal expansion (CTE_(clad)) that is greater than the CTE_(core).

According to an eighteenth aspect, a method comprises the method according to the sixteenth or seventeenth aspect, wherein the ion exchange bath comprises at least a first ion source and a second ion source that is different from the first ion source.

According to a nineteenth aspect, a method comprises the method according to any one of the sixteenth through eighteenth aspects, wherein a maximum tensile stress in the at least one cladding layer is greater than or equal to a maximum tensile stress in the at least one core layer after the contacting with the ion exchange bath.

According to a twentieth aspect, a method comprises the method according to any one of the sixteenth through nineteenth aspects, wherein a magnitude of stress at an outer surface of the glass article is greater than or equal to about 300 MPa after the contacting with the ion exchange bath.

Additional features and advantages will be set forth in the detailed description, which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description, which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a glass article in accordance with one or more embodiments shown and described herein;

FIG. 2 is a cross-sectional view of an overflow distributor that can be used for forming a glass article in accordance with one or more embodiments shown and described herein;

FIG. 3 is a plot of ion concentration of an example embodiment in which concentration is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 4 is a plot of the stress profile of the example embodiment corresponding to the plot of FIG. 3 in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 5 is a plot of ion concentration of another example embodiment in which concentration is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 6 is a plot of the stress profile of the example embodiment corresponding to the plot of FIG. 5 in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 7 is a plot of ion concentration of another example embodiment in which concentration is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 8 is a plot of the stress profile of the example embodiment corresponding to the plot of FIG. 7 in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 9 is a plot of stress profiles of example embodiments in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 10 is a plot of stress profiles of example embodiments in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 11 is a plot of stress profiles of example embodiments in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis

FIG. 12 is a representation of a non-frangible sample after a frangibility test;

FIG. 13 is a representation of a frangible sample after a frangibility test;

FIG. 14 is a plot of stress profiles of another example embodiment in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 15 is a plot of stress profiles of example embodiments in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis;

FIG. 16 is a plot of retained strength profiles of example embodiments in which retained strength is plotted on the y-axis and flaw size is plotted on the x-axis;

FIG. 17 is a plot of stress profiles of example embodiments in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis; and

FIG. 18 is a plot of retained strength profiles of the example embodiments corresponding to the stress profiles of FIG. 17 in which retained strength is plotted on the y-axis and flaw size is plotted on the x-axis.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

Directional terms as used herein—for example up, down, right, left, front, back, top, bottom—are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component includes aspects having two or more such components, unless the context clearly indicates otherwise. Also, the word “or” when used without a preceding “either” (or other similar language indicating that “or” is unequivocally meant to be exclusive—e.g., only one of x or y, etc.) shall be interpreted to be inclusive (e.g., “x or y” means one or both x or y).

All disclosed ranges are to be understood to encompass and provide support for claims that recite any and all subranges or any and all individual values subsumed by each range. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all subranges or individual values that are between and/or inclusive of the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth).

As used herein, the term “average coefficient of thermal expansion,” or “average CTE,” refers to the average coefficient of linear thermal expansion of a given material or layer between 0° C. and 300° C. As used herein, the term “coefficient of thermal expansion,” or “CTE,” refers to the average coefficient of thermal expansion unless otherwise indicated.

Compressive stress (including surface compressive stress) is measured by a refracted near-field (RNF) technique. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the birefringence of the glass. SOC in turn is measured according to Procedure C (Glass Disc Method) described in ASTM standard C770-16, entitled “Standard Test Method for Measurement of Glass Stress-Optical Coefficient,” the contents of which are incorporated herein by reference in their entirety. Depth of compression (DOC) may be measured with the RNF technique or a scattered light polariscope (SCALP) technique known in the art. The maximum central tension (CT) values are measured using a scattered light polariscope (SCALP) technique known in the art.

Concentration profiles of various constituent components in the glass, such as alkali constituent components, were measured by electron probe microanalysis (EPMA). EPMA may be utilized, for example, to discern compressive stress in the glass due to the ion exchange of alkali ions into the glass from compressive stress due to lamination.

The phrase “depth of compression” and “DOC” refer to the position in the glass where compressive stress transitions to tensile stress.

The term “formed from” can mean one or more of comprises, consists essentially of, or consists of. For example, a component that is formed from a particular material can comprise the particular material, consist essentially of the particular material, or consist of the particular material.

In various embodiments, a glass article comprises at least a first layer and a second layer. For example, the first layer comprises a core layer, and the second layer comprises one or more cladding layers 104, 106 adjacent to the core layer. The first layer and/or the second layer are glass layers comprising a glass material, a ceramic material, a glass-ceramic material, or a combination thereof. In some embodiments, the first layer and/or the second layer are transparent glass layers.

The core layer has a core CTE, and the cladding layer has a cladding CTE. For example, the core layer is formed from a first glass composition with the core CTE and the cladding layer is formed from a second glass composition with the cladding CTE. In some embodiments described herein, the core CTE is less than the cladding CTE (i.e., the cladding CTE is greater than the core CTE) such that the core layer is in compression and the cladding layer is in tension, prior to any ion exchange treatment. In some embodiments, after the glass article is subjected to an ion exchange treatment, a stress profile of the glass article increases with increasing distance from an outer surface of the cladding layer from a compressive stress to a tensile stress, transitions to a compressive stress as a step-change at an interface region between the core layer and the cladding layer, and increases with increasing distance from the interface region to a center of the core layer from the compressive stress to a tensile stress. Such “jumping” tension-compression profiles and otherwise non-conventional stress profiles in the glass article can enable deeper cracks to be arrested, as will be described in greater detail below.

FIG. 1 is a cross-sectional view of one embodiment of a glass article 100. The glass article 100 is a laminated sheet including a plurality of glass layers. In embodiments, the laminated sheet may be substantially planar, as shown in FIG. 1 , or the laminated sheet may be non-planar. The glass article 100 comprises a core layer 102 disposed between a first cladding layer 104 and a second cladding layer 106. In some embodiments, the first cladding layer 104 and the second cladding layer 106 are exterior layers relative to the core layer 102, as shown in FIG. 1 . For example, an outer surface 108 of the first cladding layer 104 serves as an outer surface of the glass article 100 and/or an outer surface 110 of the second cladding layer 106 serves as an outer surface of the glass article 100. In other embodiments, the first cladding layer 104 and/or the second cladding layer 106 are intermediate layers disposed between the core layer 102 and an exterior layer (not shown).

The core layer 102 comprises a first major surface and a second major surface opposite the first major surface. In some embodiments, the first cladding layer 104 is fused to the first major surface of the core layer 102. Additionally or alternatively, the second cladding layer 106 is fused to the second major surface of the core layer 102. In such embodiments, an interface 112 between the first cladding layer 104 and the core layer 102 and/or an interface 114 between the second cladding layer 106 and the core layer 102 are free of any bonding material such as, for example, an adhesive, a coating layer, or any non-glass material added or configured to adhere the respective cladding layers 104, 106 to the core layer 102. Thus, the first cladding layer 104 and/or the second cladding layer 106 are fused directly to the core layer 102 and/or are directly adjacent to the core layer 102. In some embodiments, the glass article 100 includes one or more intermediate layers disposed between the core layer 102 and the first cladding layer and/or between the core layer 102 and the second cladding layer. For example, the intermediate layers may comprise intermediate glass layers and/or diffusion layers formed at the interface of the core layer 102 and the cladding layer. The diffusion layer can comprise a blended region comprising components of each layer adjacent to the diffusion layer (e.g., a blended region between two directly adjacent glass layers). In some embodiments, the glass article 100 comprises a glass-glass laminate in which the interfaces between directly adjacent glass layers are glass-glass interfaces.

In some embodiments, the core layer 102 comprises a first glass composition (also referred to herein as a core glass composition) and first and/or second cladding layers 104, 106 comprise a second glass composition (also referred to herein as a clad glass composition) that is different from the first glass composition. The first glass composition and the second glass composition are different from each other prior to chemically strengthening the glass article 100 as described herein. For example, in the embodiment shown in FIG. 1 , the core layer 102 comprises the first glass composition and each of the first cladding layer 104 and the second cladding layer 106 comprises the second glass composition. In other embodiments, the first cladding layer comprises the second glass composition and the second cladding layer comprises a third glass composition that is different from the first glass composition and/or the second glass composition.

In various embodiments, the core layer 102 is formed from an ion exchangeable core glass composition and the cladding layers 104, 106 are formed from an ion exchangeable clad glass composition. In embodiments, as described above and below, the core glass composition and the clad glass composition have different coefficients of thermal expansion.

The glass article can be formed using a suitable process such as, for example, a fusion draw, down draw, slot draw, up draw, or float process. In some embodiments, the glass article 100 is formed using a fusion draw process. FIG. 2 is a cross-sectional view of an example embodiment of overflow distributor 200 that can be used to form a glass article such as, for example, the glass article 100.

The overflow distributor 200 can be configured as described in U.S. Pat. No. 4,214,886, which is incorporated by reference in its entirety. For example, the fusion draw apparatus includes a lower overflow distributor 220 and an upper overflow distributor 240 positioned above the lower overflow distributor 220. The lower overflow distributor 220 includes a trough 222. A first glass composition 224 is melted and fed into the trough 222 in a viscous state. The first glass composition 224 forms the core layer 102 of the glass article 100 as further described below. The upper overflow distributor 240 includes a trough 242. A second glass composition 244 is melted and fed into the trough 242 in a viscous state. The second glass composition 244 forms first and second cladding layers 104, 106 of the glass article 100 as further described below.

The first glass composition 224 overflows trough 222 and flows down opposing outer forming surfaces 226 and 228 of the lower overflow distributor 220. The outer forming surfaces 226 and 228 converge at a draw line 230. The separate streams of the first glass composition 224 flowing down respective outer forming surfaces 226 and 228 of the lower overflow distributor 220 converge at the draw line 230 where they are fused together to form the core layer 102 of the glass article 100.

The second glass composition 244 overflows the trough 242 and flows down opposing outer forming surfaces 246 and 248 of the upper overflow distributor 240. The second glass composition 244 is deflected outward by the upper overflow distributor 240 such that the second glass composition 244 flows around the lower overflow distributor 220 and contacts the first glass composition 224 flowing over the outer forming surfaces 226 and 228 of the lower overflow distributor 220. The separate streams of the second glass composition 244 are fused to the respective separate streams of the first glass composition 224 flowing down the respective outer forming surfaces 226 and 288 of the lower overflow distributor 220. Upon convergence of the streams of the first glass composition 224 at the draw line 230, the second glass composition 244 forms first and second cladding layers 104, 106 of the glass article 100.

In some embodiments, the first glass composition 224 of the core layer 102 in the viscous state is contacted with the second glass composition 244 of the first and second cladding layers 104, 106 in the viscous state to form the laminated sheet. In some of such embodiments, the laminated sheet is part of a glass ribbon traveling away from the draw line 230 of the lower overflow distributor 220, as shown in FIG. 2 . The glass ribbon can be drawn away from the lower overflow distributor 220 by a suitable means including, for example, gravity and/or pulling rollers. The glass ribbon cools as it travels away from the lower overflow distributor 220. The glass ribbon is severed to separate the laminated sheet therefrom. Thus, the laminated sheet is cut from the glass ribbon. The glass ribbon can be severed using a suitable technique such as, for example, scoring, bending, thermally shocking, and/or laser cutting. In some embodiments, the glass article 100 comprises the laminated sheet as shown in FIG. 1 . In other embodiments, the laminated sheet can be processed further (e.g., by cutting or molding) to form the glass article.

Although the glass article 100 is shown in FIG. 1 as including three layers, other embodiments are contemplated. For example, the glass article may have two, four, or more layers. Glass articles including two layers can be formed using two overflow distributors positioned such that the two layers are joined while traveling away from the respective draw lines of the overflow distributors or by using a single overflow distributor with a divided trough such that two glass compositions flow over opposing outer forming surfaces of the overflow distributor and converge at the draw line of the overflow distributor. Glass articles including four layers can be formed using additional overflow distributors and/or using overflow distributors with divided troughs. Thus, a glass article having a predetermined number of layers can be formed by modifying the overflow distributor accordingly.

Although the glass article 100 is shown in FIG. 1 as comprising a laminated sheet, other forms are contemplated. For example, the glass article may be in the form of a laminated tube comprising multiple tubular layers (e.g., formed by one or more annular orifices), and a partial cross-section of the laminated tube may have a laminated structure similar to that shown in FIG. 1 . In other embodiments, the glass article may be a shaped glass article, such as may be formed by shaping or molding a laminated sheet.

In some embodiments, the glass article 100 has a thickness of at least about 0.05 mm, at least about 0.1 mm, at least about 0.2 mm, or at least about 0.3 mm. Additionally or alternatively, the glass article 100 has a thickness of less than about 2 mm, less than about 1.5 mm, less than about 1 mm, less than about 0.7 mm, or less than about 0.5 mm. In some embodiments, a ratio of a thickness of the core layer 102 to a thickness of the glass article 100 is at least about 0.7, a least about 0.8, at least about 0.85, at least about 0.9, or at least about 0.95. Additionally or alternatively, the ratio of the thickness of the core layer 102 to the thickness of the glass article 100 is less than about 0.95, less than about 0.93, less than about 0.9, less than about 0.87, or less than about 0.85. In some embodiments, a thickness of the second layer (e.g., each of the first cladding layer 104 and the second cladding layer 106) is from about 0.01 mm to about 0.3 mm.

In some embodiments, the first glass composition and/or the second glass composition have a liquidus viscosity suitable for forming the glass article 100 using a fusion draw process as described herein. For example, the first glass composition of the core layer 102 may have a liquidus viscosity of at least about 100 kP, at least about 200 kP, or at least about 300 kP. Additionally or alternatively, the first glass composition comprises a liquidus viscosity of less than about 3000 kP, less than about 2500 kP, less than about 1000 kP, or less than about 800 kP. The second glass composition of the first and/or second cladding layers 104 and 106 may have a liquidus viscosity of at least about 50 kP, at least about 100 kP, or at least about 200 kP. Additionally or alternatively, the second glass composition comprises a liquidus viscosity of less than about 3000 kP, less than about 2500 kP, less than about 1000 kP, or less than about 800 kP. The first glass composition can aid in carrying the second glass composition over the overflow distributor to form the second layer. Accordingly, the second glass composition can have a liquidus viscosity that is lower than generally considered suitable for forming a single layer sheet using a fusion draw process.

In various embodiments described herein, the glass article 100 is strengthened by a combination of mechanical strengthening and chemical strengthening. For example, the glass article 100 may have a CTE mismatch as described herein, and may be chemically strengthened, such as by an ion exchange treatment, to further increase the compressive stress near the outer surface of the second layer and/or to form a compressive peak within the second layer.

In various embodiments, the glass article 100 is mechanically strengthened. For example, the second glass composition forming the first and/or second cladding layers 104, 106 may have a different CTE than the first glass composition forming the core layer 102. In particular, in some embodiments, the first and second cladding layers 104, 106 may be formed from a glass composition having a higher CTE than the glass composition of the core layer 102. The CTE mismatch (i.e., the difference between the CTE of the first and second cladding layers 104, 106 and the CTE of the core layer 102) results in the formation of compressive stress in the core layer 102 and tensile stress in the cladding layers 104, 106 upon cooling of the glass article 100. In various embodiments, each of the first and second cladding layers 104, 106, independently, can have a higher CTE, a lower CTE, or substantially the same CTE as the core layer 102. Surface compressive stresses tend to suppress existing surface flaws from developing into cracks.

In embodiments, the CTE of the core layer 102 and the CTE of the first and/or second cladding layers 104, 106 differ by at least about 5×10⁻⁷° C.⁻¹, at least about 15×10⁻⁷° C.⁻¹, at least about 25×10⁻⁷° C.⁻¹, or at least about 30×10⁻⁷° C.⁻¹. Additionally or alternatively, the CTE of the core layer 102 and the CTE of the first and/or second cladding layers 104, 106 differ by less than about 100×10⁻⁷° C.⁻¹, less than about 75×10⁻⁷° C.⁻¹, less than about 50×10⁻⁷° C.⁻¹, less than about 40×10⁻⁷° C.⁻¹, less than about 30×10⁻⁷° C.⁻¹, less than about 20×10⁻⁷° C.⁻¹, or less than about 10×10⁻⁷° C.⁻¹. For example, in some embodiments, the CTE of the core layer 102 and the CTE of the first and/or second cladding layers 104, 106 differ by about 5×10⁻⁷° C.⁻¹ to about 30×10⁻⁷° C.⁻¹, or about 5×10⁻⁷° C.⁻¹ to about 20×10⁻⁷° C.⁻¹. In some embodiments, the second glass composition of the first and/or second cladding layer has a CTE of less than about 66×10⁻⁷° C.⁻¹, less than about 55×10⁻⁷° C.⁻¹, less than about 50×10⁻⁷° C.⁻¹, less than about 40×10⁻⁷° C.⁻¹, or less than about 35×10⁻⁷° C.⁻¹. Additionally or alternatively, the second glass composition of the first and/or second cladding layer has a CTE of at least about 10×10⁻⁷° C.⁻¹, at least about 15×10⁻⁷° C.⁻¹, at least about 25×10⁻⁷° C.⁻¹, or at least about 30×10⁻⁷° C.⁻¹. The first glass composition of the core layer 102 may have a CTE of at least about 40×10⁻⁷° C.⁻¹, at least about 50×10⁻⁷° C.⁻¹, at least about 55×10⁻⁷° C.⁻¹, at least about 65×10⁻⁷° C.⁻¹, at least about 70×10⁻⁷° C.⁻¹, at least about 80×10⁻⁷° C.⁻¹, or at least about 90×10⁻⁷° C.⁻¹. Additionally or alternatively, the first glass composition of the core layer may have a CTE of less than about 120×10⁻⁷° C.⁻¹, less than about 110×10⁻⁷° C.⁻¹, less than about 100×10⁻⁷° C.⁻¹, less than about 90×10⁻⁷° C.⁻¹, less than about 75×10⁻⁷° C.⁻¹, or less than about 70×10⁻⁷° C.⁻¹.

In various embodiments described herein, the glass article 100 is chemically strengthened. For example, the glass article 100 may be subjected to an ion exchange treatment to increase the compressive stress in a region of the glass article at an outer surface of the glass article (e.g., an outer portion of the cladding layer). In some embodiments, the ion exchange treatment comprises applying an ion exchange medium to one or more surfaces of the glass article 100. The ion exchange medium may be a solution, a paste, a gel, or another suitable medium comprising larger ions to be exchanged with smaller ions in the glass (e.g., the glass of the second layer). The terms “larger ions” and “smaller ions” are relative terms, meaning that the larger ions are relatively large compared to the smaller ions and the smaller ions are relatively small compared to the larger ions. Thus, the larger ions have a larger ionic radius than the smaller ions, and the smaller ions have a smaller ionic radius than the larger ions. In some embodiments, the cladding layer of the glass article 100 comprises an alkali aluminosilicate glass. Thus, the smaller ions in the cladding layer of the glass article and the larger ions in the ion exchange medium may be monovalent alkali metal cations (e.g., Li⁺, Na⁺, and/or K⁺). Alternatively, monovalent cations in the glass article 100 may be replaced with monovalent cations other than alkali metal cations (e.g., Ag⁺ or the like). In some embodiments, the cladding layer of the glass article 100 comprises an alkaline earth aluminosilicate glass. Thus, the smaller ions in the cladding layer of the glass article and the larger ions in the ion exchange medium may be divalent alkaline earth cations (e.g., Be²⁺, Mg²⁺, Ca²⁺, and/or Sr²⁺). In some embodiments, the ion exchange medium comprises a molten salt solution, and the ion exchange treatment comprises immersing the laminated glass article in a molten salt bath comprising larger ions (e.g., K⁺, Na⁺, Sr²⁺, and/or Ca²⁺) to be exchanged with smaller ions (e.g., Na⁺, Li⁺, Ca²⁺, and/or Mg²⁺) in the glass. In some embodiments, the molten salt bath comprises a salt (e.g., a nitrate, a sulfate, and/or a chloride) of the larger ions. For example, the molten salt bath may include molten KNO₃, molten NaNO₃, or a combination thereof. Additionally or alternatively, the temperature of the molten salt bath may be from about 380° C. to about 450° C., and an immersion time is from about 2 hours to about 16 hours.

By replacing smaller ions in the glass with larger ions at the surface of the glass article 100, the compressive stress of the cladding layer is increased at the outer surface of the glass article. For example, during the ion exchange treatment, the larger ions from the ion exchange medium diffuse into an outer portion of the cladding layer of the glass article 100 and the smaller ions from the glass diffuse out of the outer portion of the cladding layer of the glass article. Thus, the outer portion of the cladding layer comprises an exchanged region of the glass article. The increased concentration of the larger ions in the ion exchanged region causes crowding of the glass and increases the compressive stress of the glass article 100 in the ion exchanged region. In some embodiments, subjecting the glass article 100 to the ion exchange treatment increases a surface compressive stress at the outer surface of the glass article (e.g., from an initial surface compressive stress generated by the CTE mismatch) to a final compressive stress value. For example, the final compressive stress value is at least about 200 MPa, at least about 300 MPa, at least about 400 MPa, at least about 500 MPa, at least about 600 MPa, at least about 700 MPa, at least about 800 MPa, at least about 900 MPa, or at least about 1,000 MPa. Additionally or alternatively, the final compressive stress value is less than about 1300 MPa, less than about 1200 MPa, less than about 1000 MPa, less than about 900 MPa, less than about 800 MPa or less than about 600 MPa.

In various embodiments herein, the core layer 102 is ion exchange strengthened through the cladding layers 104, 106. The core layer 102 and the cladding layers 104, 106 have different compositional characteristics to achieve these properties. The compositional characteristics of each glass composition produces a unique alkali ion diffusion profile through the thickness of the laminated glass article after strengthening by ion exchange.

Prior to ion exchange strengthening, the concentration of various constituent components of the glass network (e.g., glass formers such as SiO₂ and B₂O₃, intermediates such as Al₂O₃, and modifiers such as CaO, Na₂O, etc.) of the cladding layers 104, 106 are generally uniformly distributed from the outer surfaces of the laminated glass article to the respective interface regions. For example, the cladding layers 104, 106 comprise at least one glass former and a concentration of the glass former is substantially constant from the outer surface of the glass article to the interface between the cladding layer and the core layer 102. In addition, the cladding layers 104, 106 comprise at least one modifier, such as Na₂O and/or another alkali oxide, and a concentration of the modifier is substantially constant from the outer surface of the glass article to the interface between the cladding layer and the core layer 102.

Similarly, the concentration of various constituent components of the glass network (e.g., glass formers such as SiO₂ and B₂O₃, intermediates such as Al₂O₃, and modifiers such as CaO, Na₂O, etc.) of the core layer 102 are generally uniformly distributed from the respective interface to the centerline of the core layer 102. The term “centerline” as used herein, refers to the midpoint of the thickness of the core layer 102. For example, the core layer 102 comprises at least one glass former and a concentration of the glass former is substantially constant from a core side of the interface to the centerline of the core layer 102. In addition, the core layer 102 comprises at least one modifier, such as Na₂O and/or another alkali oxide, and a concentration of the glass modifier is substantially constant from a core side of the interface to the centerline of the core layer 102.

However, after ion exchange, the concentration of the alkali oxides, such as K₂O, varies in both the core layer 102 and the cladding layers 104, 106 as a function of depth from the surfaces 108, 110 of the glass article 100, as shown in FIG. 3 . Specifically, a concentration of K₂O 302 decreases from the surfaces 108, 110 of the glass article 100, through the cladding layers 104, 106, to a cladding side of the interfaces 112, 114. That is, the concentration of K₂O 302 in the cladding layers 104, 106 as a function of distance from the surfaces 108, 110 has a negative slope. Similarly, the concentration of K₂O 302 decreases from a core side of the interface in the direction of the centerline CL of the core layer 102.

Additionally, the concentration of Na₂O 304 increases from the surfaces 108, 110 of the glass article 100, through the cladding layers 104, 106 to a cladding side of the interfaces 112, 114. That is, a concentration of Na₂O 304 in the cladding layers 104, 106 as a function of distance from the surfaces 108, 110 has a positive slope. The Na₂O concentration 304 then changes as a step change at the interface between the cladding layers 104, 106 and the core layer 102, and then continuously decreases within the core layer 102.

Moreover, in FIG. 3 , the Li₂O concentration 306 increases with increasing distance from the surface of the cladding layers 104, 106 to a centerline CL in the core layer 102 from a surface concentration of zero to a maximum Li₂O concentration 306 at the centerline.

While the concentration of alkali oxides in the cladding layers 104, 106 changes as a result of ion exchange strengthening, it should be understood that the concentrations of the other constituents of the glass network (i.e., glass formers, intermediates, and non-mobile modifiers such as alkaline earth oxides (CaO, MgO, etc.)) remain substantially the same (i.e., substantially uniform through the thicknesses of the cladding layers 104, 106 and substantially uniform through the thickness of the core layer 102).

The ion exchange treatment also creates a unique stress profile, one example of which is graphically depicted in FIG. 4 . As shown in FIG. 4 , the glass cladding layer has a stress profile due to ion exchange. The stress in the region from the surface of the glass up to the core-clad interface at 100 μm includes compressive stresses and tensile stresses due to ion exchange strengthening. The core layer 102 (i.e., the portion of the laminate at depths greater than 100 μm) also includes regions that are compressively stressed and regions that are under tensile stress due to ion exchange strengthening the cladding layer through the core layer 102. In the example shown in FIG. 4 , the stress profile 402 corresponds to a profile for glass chemically strengthened using a dual ion exchange treatment as described herein.

More particularly, in FIG. 4 , the stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from a compressive stress to a tensile stress, and then transitions to a compressive stress as a step-change at an interface region between the core layer 102 and the cladding layer. The stress of the core layer 102 increases with an increasing distance from the interface region to a center of the core layer 102 from the compressive stress to a tensile stress.

The alkali concentration profile of FIG. 3 and the stress profile of FIG. 4 were obtained using lithium-containing cladding layers 104, 106 and a sodium-containing core layer 102. FIGS. 5-8 each show an alkali concentration profile (FIGS. 5 and 7 ) or a stress profile (FIGS. 6 and 8 ) of a glass laminate including a lithium-containing core layer 102 and sodium-containing cladding layers 104, 106. The profiles in FIGS. 5 and 6 were obtained using a cladding layer thickness of 70 μm, while the profiles in FIGS. 7 and 8 were obtained using a cladding layer thickness of 25 μm.

As with the previous example, following ion exchange, the concentration of the alkali oxides, such as K₂O, varies in both the core layer 102 and the cladding layers 104, 106 as a function of depth from the surfaces 108, 110 of the glass article 100, as shown in FIGS. 5 and 7 . Specifically, a concentration of K₂O 502, 702 decreases from the surfaces 108, 110 of the glass article 100, through the cladding layers 104, 106, to a cladding side of the interfaces 112, 114. That is, the concentration of K₂O 502, 702 in the cladding layers 104, 106 as a function of distance from the surfaces 108, 110 has a negative slope. Similarly, the concentration of K₂O 502, 702 decreases from a core side of the interface in the direction of the centerline CL of the core layer 102.

Additionally, in FIGS. 5 and 7 , the Li₂O concentration 504, 704 increases with increasing distance from the surface of the cladding layers 104, 106 to a centerline in the core layer 102 from a surface concentration of 0 to a maximum Li₂O concentration 504, 704 at the centerline CL.

However, in FIGS. 5 and 7 , the concentration of Na₂O 506, 706 increases from the surfaces 108, 110 of the glass article 100, through the cladding layers 104, 106 to a cladding side of the interfaces 112, 114. That is, a concentration of Na₂O 506, 706 in the cladding layers 104, 106 as a function of distance from the surfaces 108, 110 has a positive slope. The Na₂O concentration 506, 706 then changes as a step change at the interface between the cladding layers 104, 106 and the core layer 102, where it initially increases and then decreases within the core layer 102.

While the concentration of alkali oxides in the cladding layers 104, 106 changes as a result of ion exchange strengthening, it should be understood that the concentrations of the other constituents of the glass network (i.e., glass formers, intermediates, and non-mobile modifiers such as alkaline earth oxides (CaO, MgO, etc.)) remain substantially the same (i.e., substantially uniform through the thicknesses of the cladding layers 104, 106 and substantially uniform through the thickness of the core layer 102).

In the stress profiles depicted in FIGS. 6 and 8 , the cladding layers 104, 106 have profiles due to ion exchange. In FIG. 6 , the stress in the region from the surface of the glass up to the core-cladding interface at 70 μm includes compressive stresses and tensile stresses due to ion exchange strengthening. The core layer 102 (i.e., the portion of the laminate at depths greater than 70 μm) also includes regions that are compressively stressed and regions that are under tensile stress due to ion exchange strengthening the core layer 102 through the cladding layers 104, 106. In the example shown in FIG. 6 , the stress profile 602 corresponds to a profile for glass chemically strengthened using a dual ion exchange treatment as described herein.

More particularly, in FIG. 6 , the stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from a compressive stress to a tensile stress, and then transitions to a compressive stress as a step-change at an interface region between the core layer 102 and the cladding layer. The stress of the core layer 102 increases with an increasing distance from the interface region to a center of the core layer 102 from the compressive stress to a tensile stress.

However, in FIG. 8 , the stress profile 802 indicates that the stress in the region from the surface of the glass up to the core-cladding interface at 25 μm includes only compressive stresses due to ion exchange strengthening. The core layer 102 (i.e., the portion of the laminate at depths greater than 25 μm) includes regions that are compressively stressed and regions that are under tensile stress due to ion exchange strengthening the core layer 102 through the cladding layers 104, 106.

More particularly, in FIG. 8 , the stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from an initial compressive stress to a minimum compressive stress, and then transitions to a maximum compressive stress as a step-change at an interface region between the core layer 102 and the cladding layer. The stress of the core layer 102 increases with an increasing distance from the interface region to a center of the core layer 102 from the compressive stress to a tensile stress.

Notably, the reduction in the cladding layer thickness eliminates the tensile stress in the cladding layer, reduces the central tension in the glass article, and maintains a depth of compression (DOC) of about 100 μm, or about 21% of the total thickness of the glass laminate. In other words, high levels of compression can be generated without generating significant tension within the core layer 102.

The retained strength of a glass article can be determined based on the stress profile of the glass article. For example, the retained strength is determined by forming a flaw extending from a surface of the glass article to a specified depth and then determining the strength of the glass article after formation of the flaw. The strength is flexural strength of the glass article determined using, for example, a ring-on-ring test method (e.g., as described in ASTM C1499-09), a ball-on-ring test method, a three-point bend test method, a four-point bend test method, or another suitable method or technique. Such a retained strength determination can be conducted using a fracture mechanics simulation based on the stress profile of the glass article.

Moreover, the improved resistance to breakage resulting from large flaws can be achieved by the strength profile 802 while decreasing the maximum tensile stress of the tensile region as compared to the retained strength profile 602 and/or the retained strength profile 402. For example, maintaining the compressive stress at a relatively constant level relatively deep into the compressive region (e.g., over the intermediate portion) can help to maintain the area under the compressive portion of the stress profile curve, which is proportional to the maximum tensile stress in the tensile region, relatively low while also providing protection against breakage caused by relatively deep flaws. Thus, the maximum tensile stress can be maintained below the frangibility limit.

In various embodiments, the ion exchange parameters and glass compositions selected for each of the cladding layers 104, 106 and the core layer 102 can be selected to achieve a particular stress profile. Various example stress profiles are depicted in FIG. 9 .

For example, stress profile 902 can be obtained where the stress gradient in the deeper portion of the cladding layer is not continuous. Rather, the stress in the cladding layer monotonically changes from the surface to the clad-core interface, whereupon it jumps to compression within the core layer 102. Alternatively, the stress may have a gradient across the clad-core interface, taking into account that an inter-diffusion layer exists between the cladding layer and the core layer 102, in which the composition gradually changes from the clad glass composition to the core glass composition. In one particular embodiment, the stress profile 902 can be achieved by using a core glass composition that has a much higher alkali content than the clad glass composition.

For example, in one particular embodiment, the clad glass composition includes 6 mol % Na₂O and 1 mol % K₂O as the alkali, and the core glass composition includes 18 mol % Na₂O and 3 mol % K₂O. After ion exchanging the sample as a first step in a bath having 50% NaNO₃ and 50% KNO₃ for a first exchange time of, for example and without limitation, from greater than or equal to 1 hour to less than or equal to 100 hours, there is substantial penetration of K ions into the core glass B. Then, in a second step, the sample is ion exchanged in a bath having 100% KNO₃, for a second exchange time that is less than the first exchange time that is adequate for the K ions to produce a gradient profile within the cladding layer. The second exchange time may be, for example and without limitation, from about 10 minutes to less than or equal to 5 hours so long as the second exchange time is less than the first exchange time.

As another example, the stress profile 904 can be obtained after a first long ion exchange step (e.g., after an ion exchange time of, for example and without limitation, from greater than or equal to 1 hour to less than or equal to 100 hours), but the second step is an ion exchange step with non-permeable boundary conditions in which only ions already in the glass diffuse between the core layer and the cladding layer, but no additional ions are diffused into the glass article from an external source (e.g., a molten salt bath). The second ion exchange step may be performed, for example and without limitation, by heating the glass article to a temperature sufficient for ion exchange (e.g., from about 350° C. to about 550° C.). This increases the ion exchange between the core layer and the cladding layer(s) and results in increased compressive stress at the interface between the core layer and the cladding layer(s). However, due to force balancing, the compressive stress at or near the surface of the cladding layer decreases. As a result, the stress profile 904 in the cladding layer is substantially linear. In such embodiments, the second derivative of the stress profile can be neglected for purposes of distinguishing the mechanical performance.

In still another example, the stress profile 906 can be obtained by using a clad glass composition with a much higher alkali diffusivity than the core glass composition, so the stress gradient in the cladding layer(s) is substantially eliminated. In this embodiment, the stress gradient in the cladding layer(s) is sufficiently reduced that it is irrelevant for purposes of distinguishing the mechanical performance. The glass laminate is subjected to a first, long ion exchange (e.g., an ion exchange time of, for example and without limitation, from greater than or equal to 1 hour to less than or equal to 100 hours), so that the stress profile penetrates all the way to the core layer 102, followed by a heat treatment to produce the flat region near the surface of the cladding layer.

The stress profile 908 can be obtained by using a clad glass composition with a much higher diffusivity than the core glass composition (but a lower diffusivity than the clad glass composition used to obtain the stress profile 906) in conjunction with a long ion exchange as a first step, so the stress profile penetrates all the way to the core layer 102, followed by a heat treatment to produce the flat region of the stress profile near the surface of the cladding layer.

In various embodiments, enabling ion exchange along the clad-core interface can also result in various stress profiles. For example, as shown in FIG. 10 , the amount of time that a sample is subjected to an ion exchange process can change the resultant stress profile. In FIG. 10 , glass laminates having a sodium rich clad glass composition and a potassium rich core glass composition were ion exchanged for various amounts of time. In particular, stress profile 1002 was obtained following an ion exchange step of a first duration D1 (hours), stress profile 1004 was obtained following an ion exchange step of approximately 4×D1, stress profile 1006 was obtained following an ion exchange step of approximately 15×D1, and stress profile 1008 was obtained following an ion exchange step of 25×D1. Although the stresses from the CTE mismatch between the cladding layer and the core layer 102 are not included in the stress profiles depicted in FIG. 10 , the CTE stresses would shift the stresses within the cladding layer in the −Y direction while the CTE stresses would shift the stresses within the core layer 102 in the +Y direction. Accordingly, with this particular combination of alkalis in the core and clad glass compositions, a thicker cladding layer can be employed without introducing tensile stress to the cladding layer.

Still other stress profiles can be obtained by reversing the alkali content of the cladding layer and core layer 102. For example, as shown in FIG. 11 , glass laminates having a potassium rich clad glass composition and a sodium rich core glass composition were ion exchanged for various amounts of time. In particular, stress profile 1102 was obtained following an ion exchange step of a first duration D1 (hours), stress profile 1104 was obtained following an ion exchange step of approximately 4×D1, stress profile 1106 was obtained following an ion exchange step of approximately 15×D1, and stress profile 1108 was obtained following an ion exchange step of approximately 25×D1. Although the stresses from the CTE mismatch between the cladding layer and the core layer 102 are not included in the stress profiles depicted in FIG. 11 , the CTE stresses would shift the stresses within the cladding layer in the +Y direction while the CTE stresses would shift the stresses within the core layer 102 in the −Y direction. Accordingly, with this particular combination of alkalis in the core and clad glass compositions, a thicker cladding layer can be employed to reduce the central tension within the glass laminate. Moreover, the combination of glass compositions along with the CTE mismatch can result in compressive stresses in the core layer 102.

Various embodiments described herein can also be used to reduce the frangibility of a glass laminate. In some embodiments, the glass articles disclosed herein are not frangible upon being ion-exchanged. Frangible behavior refers to specific fracture behavior when a glass article is subjected to an impact or insult. As utilized herein, a glass is considered non-frangible when it exhibits at least one of the following in a test area as the result of a frangibility test: (1) four or less fragments with a largest dimension of at least 1 mm, and/or (2) the number of bifurcations is less than or equal to the number of crack branches. The fragments, bifurcations, and crack branches are counted based on any 2 inch by 2 inch square centered on the impact point. Thus, a glass is considered non-frangible if it meets one or both of tests (1) and (2) for any 2 inch by 2 inch square centered on the impact point where the breakage is created according to the procedure described below. In a frangibility test, an impact probe is brought in to contact with the glass, with the depth to which the impact probe extends into the glass increasing in successive contact iterations. The step-wise increase in depth of the impact probe allows the flaw produced by the impact probe to reach the tension region while preventing the application of excessive external force that would prevent the accurate determination of the frangible behavior of the glass. In one embodiment, the depth of the impact probe in the glass may increase by about 5 μm in each iteration, with the impact probe being removed from contact with the glass between each iteration. The test area is any 2 inch by 2 inch square centered at the impact point. FIG. 12 depicts a non-frangible test result. As shown in FIG. 12 , the test area is a square that is centered at the impact point 1230, where the length of a side of the square a is 2 inches. The non-frangible sample shown in FIG. 12 includes three fragments 1242, and two crack branches 1240 and a single bifurcation 1250. Thus, the non-frangible sample shown in FIG. 12 contains less than 4 fragments having a largest dimension of at least 1 mm and the number of bifurcations is less than or equal to the number of crack branches. As utilized herein, a crack branch originates at the impact point, and a fragment is considered to be within the test area if any part of the fragment extends into the test area. While coatings, adhesive layers, and the like may be used in conjunction with the strengthened glass articles described herein, such external restraints are not used in determining the frangibility or frangible behavior of the glass articles. In some embodiments, a film that does not affect the fracture behavior of the glass article may be applied to the glass article prior to the frangibility test to prevent the ejection of fragments from the glass article, increasing safety for the person performing the test.

A frangible sample is depicted in FIG. 13 . The frangible sample includes 5 fragments 1242 having a largest dimension of at least 1 mm. The sample depicted in FIG. 13 includes 2 crack branches 1240 and 3 bifurcations 1250, producing more bifurcations than crack branches. Thus, the sample depicted in FIG. 13 does not exhibit either four or less fragments or the number of bifurcations being less than or equal to the number of crack branches.

In the frangibility test described herein, the impact is delivered to the surface of the glass article with a force that is just sufficient to release the internally stored energy present within the strengthened glass article. That is, the point impact force is sufficient to create at least one new crack at the surface of the strengthened glass sheet and extend the crack through the compressive stress CS region (i.e., depth of compression) into the region that is under central tension CT.

Accordingly, the chemically strengthened glasses described herein are “non-frangible”—i.e., they do not exhibit frangible behavior as described hereinabove when subjected to impact by a sharp object.

Frangibility of the chemically strengthened glasses described herein can be reduced by burying a compressive stress peak within the core layer 102. For example, a glass article having a thin core layer 102 that is low in potassium concentration and thick cladding layers 104, 106 that include potassium ions can have a stress profile 1402, as shown in FIG. 14 . When the glass article is subjected to an ion exchange treatment, the potassium ions from the cladding layer exchange into the core layer 102, increasing the compression within the core layer 102. Without being bound by theory, it is believed that the thinner the core layer 102 and the stronger the CTE mismatch, the higher the compressive stress barrier can be built into the glass. This compression reduces frangibility by dividing the tensile region, and arrests cracks.

As described hereinabove, in various embodiments, the glass article is mechanically strengthened through a CTE mismatch between the core layer 102 and the cladding layers 104, 106. FIG. 15 depicts various stress profiles accounting for the dual ion exchange process described herein and the CTE difference.

In FIG. 15 , stress profiles 1502 and 1506 correspond to glass articles that were subjected to a dual ion exchange process as described herein, but are not laminated. Stress profiles 1504 and 1508 correspond to laminated glass articles that were subjected to the same dual ion exchange processes that generated stress profiles 1502 and 1506, respectively. The glass article corresponding to stress profile 1508 has a lower laminate CTE difference, a lower peak compressive stress, and a deeper depth of compression for the ion exchange and the step profile as compared to the glass article corresponding to stress profile 1504. As shown in FIG. 15 , the stress profiles 1504 and 1508 for the mechanically and chemically strengthened glass articles provide good protection against both short and deep flaws, and the addition of the laminate, and particularly the constant stress profile region, enables higher stress at the depth of compression instead of the error function decay that results in a region of low stress before the depth of compression. Additionally, the dual ion exchange enables the stress profile to have a non-constant stress as a function of depth.

FIG. 16 illustrates the calculated retained strength based on fracture mechanics as a function of flaw size for the glass articles corresponding to the stress profiles in FIG. 15 . In particular, retained strength profile 1602 corresponds to stress profile 1502, retained strength profile 1604 corresponds to stress profile 1504, retained strength profile 1606 corresponds to stress profile 1506, and retained strength profile 1608 corresponds to stress profile 1508. In FIG. 16 , it can be seen that the mechanically and chemically strengthened glass articles (corresponding to strength profiles 1604 and 1608) exhibit greater retained strength for flaw sizes of up to about 90 μm. Additionally, it can be seen that the glass article corresponding to strength profile 1608 has reduced strength for small flaws, but has more strength for deeper flaws when compared to the glass article corresponding to strength profile 1604. Accordingly, the dual ion exchange profile and laminate stresses can be combined to provide a desired retained strength for a given critical flaw size range.

Various additional stress profiles can be obtained in various embodiments described herein are shown in FIG. 17 . In particular, the profiles in FIG. 17 illustrate that two general strategies for leveraging the step profile obtained from a glass article having a CTE mismatch, as described herein. First, stress profile 1704 illustrates that a high compressive stress, but low thickness, cladding layers 104, 106 can be used to compensate for low compressive strength in a parabolic stress profile, such as stress profile 1702, 1708, or 1710. This strategy could result in good scratch performance resulting from a high compressive stress and high free volume due to the high fictive temperature of the glass. In particular, high free volume (e.g., low density) may result in improved scratch and/or indentation fracture resistance performance. Second, as can be seen by comparing stress profiles 1704, 1706, and 1712 to stress profiles 1702, 1708, and 1710, a high depth of compression as well as a low slope as the compressive stress transitions to a tensile stress can result in improved drop performance. By mechanically strengthening the glass article, both features can be leveraged by increasing the depth of compression via addition of a low compressive stress step profile, which extends to or beyond the depth of compression of the combined profiles. The compressive stress can be effectively “hidden” in a region where its presence does not affect drop performance.

FIG. 18 , which illustrates the retained strength of the glass articles corresponding to the stress profiles of FIG. 17 , demonstrates that both strategies are effective in leveraging the step profile. In addition, as shown in FIG. 18 , the thickness of the cladding layer can be adjusted to provide improved retained strength for any flaw range of interest. The residual strength profiles in FIG. 18 further illustrate that burying the central tension increases the retained strength while the high compressive stress increases the strength for short flaws.

Various embodiments described herein enable improved glass performance, such as scratch and crack resistance, through mechanical and chemical strengthening. Moreover, the selection of various glass compositions to attain particular CTE mismatches between the core layer and cladding layers as well as ion exchange of the core layer through the cladding layers can produce a glass article having a series of compression-tension zones through the thickness of the glass article, which can reduce frangibility and arrest cracks.

It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A laminated glass article comprising: a core layer comprising a core glass composition; and a cladding layer directly adjacent to the core layer and comprising a clad glass composition; wherein: a concentration of Li ions increases with increasing distance from an outer surface of the cladding layer to a centerline of the core layer; a concentration of K ions decreases with increasing distance from the outer surface of the cladding layer to an interface between the cladding layer and the core layer; and a concentration of Na ions increases with increasing distance from the outer surface of the cladding layer to a maximum concentration of Na ions in the cladding layer or at the interface between the cladding layer and the core layer and transitions to a lower concentration of Na ions relative to the maximum concentration of Na ions via a step change at the interface between the cladding layer and the core layer.
 2. The laminated glass article according to claim 1, wherein the concentration of Na ions decreases with increasing distance from the interface between the cladding layer and the core layer to the centerline of the core layer.
 3. The laminated glass article according to claim 1, wherein the concentration of Na ions increases and then decreases with increasing distance from the interface between the cladding layer and the core layer to the centerline of the core layer.
 4. The laminated glass article according to claim 1, wherein the concentration of Li ions increases according to a non-linear relationship.
 5. The laminated glass article according to claim 1, wherein a concentration of K ions in the core layer is zero.
 6. The laminated glass article according to claim 1, wherein the concentration of Na ions increases from the outer surface of the cladding layer to the maximum concentration of Na ions according to a linear relationship.
 7. The laminated glass article according to claim 1, wherein the concentration of Na ions increases from the outer surface of the cladding layer to the maximum concentration of Na ions according to a non-linear relationship.
 8. A laminated glass article comprising: a core layer comprising a core glass composition; and a cladding layer directly adjacent to the core layer and comprising a clad glass composition; wherein a stress of the cladding layer increases with increasing distance from an outer surface of the cladding layer from a first compressive stress to a second compressive stress, transitions to a third compressive stress as a step-change at an interface region between the core layer and the cladding layer, and increases with increasing distance from the interface region to a center of the core layer from the third compressive stress to a tensile stress, and wherein the first compressive stress is greater than the second compressive stress and lower than the third compressive stress. 